This invention relates to a method of detecting a condition associated with a final phase of a plasma dicing process. The invention relates also to associated apparatus.
Plasma dicing is a well known technique in which dies are singulated using a plasma etch process. Typically, the dies are formed from a semiconductor material such as silicon and are used in electronic applications. The process of plasma dicing can generate a significant amount of heat due to exothermic chemical reactions, ion bombardments and emission from the plasma. Additionally, it is usual to heat the plasma etch chamber to an elevated temperature, typically around 60° C., to ensure that by-products are volatile. The volatile by-products can then be pumped away. In particular, in silicon etching using a fluorine based plasma, the enthalpy of formation of the reaction Si+4F→SiF4 is 1.615 MJ/mol. This is extremely exothermic and results in a significant heat load.
The dicing of other semiconductor substrates can have high associated thermal budgets as well. For example, the dicing of GaAs wafers using chlorine based plasma chemistry can have a high thermal budget due to the continuous RF power applied to the wafer during the dicing process. For a 150 mm diameter wafer, this can be of the order of several hundred Watts. This heat load is in addition to the positive enthalpy of formation of gallium chloride (211 kJ/mol) and arsenic chloride (123 kJ/mol).
Because of the high temperatures associated with the plasma dicing process, it is usual to cool the substrate. Typically, the substrate is clamped to a cooled platen using electrostatic or mechanical methods or a combination of both methods. It is common to the mount the substrate on a frame using tape. The tape that is used for this purpose is typically formed from a polymeric material having one or more adhesive layers formed thereon. Care has to be taken to keep the polymeric mounting tape cool to avoid deformation or ‘burning’. ‘Burning’ can occur when the tape is heated beyond its thermoplastic (glass) transition point, or when a chemical reaction occurs between the tape and active species in the plasma, or when the tape decomposes into constituent polymeric compounds. In the latter case some of the constituent polymeric compounds may change phase into liquid form. Additionally, the heat load can cause out-gassing, which creates a pocket of trapped gas underneath the tape which may appear as a blister and initiate the ‘burning’ state. Therefore, the damage caused by tape deformation and ‘burning’ can be quite substantial. Significant damage to both the front side and the back side of the taped frame can occur during the plasma dicing process. Furthermore, outgassing increases the pressure in the wafer backside, and this can cause the wafer to declamp. A declamped wafer loses thermal contact with the temperature controlled surface of the platen and overheats rapidly. This can cause the tape to melt. Declamping from the platen can be very difficult to detect using conventional methods such as helium coolant gas leakage or capacitative sensing. In the case of a thinned, possibly singulated wafer, with no mechanical stiffness, localised declamping is possible.
It is known to monitor optical emission in order to determine the plasma dicing end point. It is also known to use the end point detection of the singulation breakthrough to automatically adjust the etching process to a less energetic etch rate or a ‘soft landing’ mode. However, this approach has a fundamental flaw. This is because the optical emission end point technique can only signal the singulation event itself. This too late in the process to properly prevent overheating of the tape, because the high temperatures that can cause the burning of the tape have already occurred earlier in the dicing process.
One solution to this problem is to adjust the etch to a less energetic, lower etch rate or ‘soft landing’ mode prior to the end point. The point at which the adjustment is made is estimated, based on the nominal wafer thickness and expected etch rate. The lower etch rate is then used through to the singulation end point. The singulation can be optically detected and further automatic process adjustment can be applied as necessary. This approach is not particularly satisfactory because it based on estimates. In particular, this approach is less than ideal if the wafer thickness is not consistent. If the wafer thickness is not consistent, then the etch times will vary and this will result in the estimated point at which the ‘soft landing’ mode is activated being inaccurate. For example, for a typical dicing after grind (DAG) application, the bulk silicon etch rate will be of the order of 22 microns/min, the low etch rate will be around 16 microns/min and the ‘soft landing’ etch rate will be about 6 microns/min. For a 100 micron thick wafer set to etch at 22 microns/min to a target depth of 95 microns before switching to a etch rate of 6 microns/min, the plasma dicing process will take approximately 5.2 minutes. However, if the wafer-to-wafer thickness variation is 105±5 microns the thickest wafer will take 6.8 minutes to etch. If the point at which the soft landing mode is activated is set to ensure that the thinnest wafers can be treated satisfactorily, then it follows that the thickest wafer will be subject to an extended period of time during which a low etch rate is used. This is undesirable from a manufacturing perspective. In the back-end electronic packaging industry, there are many application types and wafers of many different thicknesses are processed. A system that can etch at a high rate until the end point is approached would be extremely desirable.